In image-formation apparatuses, such as photocopiers and printers, to which electrophotographic technologies are applied, many organic photoreceptors are employed, each of which has a sensitivity to light emitted from a light-source in the apparatus within a range of wavelength of the light. It is known that the organic photoreceptor includes monolayer type photoreceptors and multilayer type photoreceptors. The monolayer type photoreceptor comprises a single photoreceptor layer wherein a charge generating material and a charge transferring material have been dispersed in a thin film of an appropriate binder resin. The multilayer type photoreceptor comprises a charge generating layer comprising a charge generating material; and a charge transferring layer comprising a charge transferring material, wherein the charge generating layer and the charge transferring layer are laminated.
Conventional phthalocyanine compound has a spectral sensitivity to a light having a long wavelength or less, good charge generation efficiency, good fastness, high sensitivity and high durability. Therefore, various phthalocyanine compounds are employed as charge generating materials. Among others, titanylphthalocyanine can be used in an electrophotographic photoreceptor, since the titanylphthalocyanine has characteristics such as high charging amount and high sensitivity, and the titanylphthalocyanine can be easily formed into a thin film, such as a charge generating layer, by vapor deposition or dispersing.
These phthalocyanine compounds have considerably different electric characteristics depending on each stacking state even though they have the same molecular structure. Stacking state of the organic compound molecule is determined by polymorph. Therefore, different polymorph provides different stacking state. Accordingly, perturbation of electron in π electron system is altered. This is why that the characterists in an electronic material such as an organic photoreceptor can be significantly altered.
Generally, titanylphthalocyanines can be produced by an urea method (Weiller method) or a phthalonitrile method. These conventional methods, however immediately after the synthesis, often provide a crude titanylphthalocyanines containing various polymorphs, such as “β-type”, “α-type” and other metastable polymorphs, i.e., a polymorphic mixture. Since the polymorphic mixture is a mixture of crystals each of which has different electric characteristics, the conventional titanylphthalocyanines are not suitable for applying to a charge generating material such as an electrophotographic photoreceptor.
Titanylphthalocyanine having a suitable single polymorph has been investigated in order to use the titanylphthalocyanine as a crude in a charge generating material. Examples of such trials include the follows.
Patent Literature 1: JP-B-7-91486 discloses a crystalline oxytitanium phthalocyanine, i.e., titanylphthalocyanine crystal, which has a maximum diffraction peak at Bragg angle (2θ±0.2°) of 27.3°, in X-ray diffraction spectrum, and other diffraction peaks at Bragg angles of 7.4°, 9.7° and 24.2°. In addition,
Patent Literature 1 further discloses that the titanylphthalocyanine is used as a charge generating material, and the charge generating material has good characteristics such as sensitivity, chargeablitiy, dark decay and residual potential.
Patent Literature 2: JP-A-63-37163 discloses a method for transforming a crystal form, which includes a step of pulverizing a B-type crystal of an oxytitanium phthalocyanine, in the presence of an A-type crystal thereof, in an organic solvent to transform the B-type crystal of the oxytitanium phthalocyanine into A-type crystal thereof.
Patent Literature 3: JP-A-9-87540 discloses a method for producing a titanylphthalocyanine crystal, which includes a step of contacting a dihalogenotitanium phthalocyanine with an acid having pKa of 5 or less, and then, in the presence of water, with an organic solvent having a specific inductive capacity of 20 or less. Crystal form of TiOPc is a so-called D-type crystal form. The Patent Literature 3 also discloses the crystal form has strong diffraction peaks at Bragg angles (2θ±0.2°) of 9.7°, 24.1° and 27.3°, in powder X-ray diffraction spectrum with CuK α-ray, and usually has a maximum diffraction peak at Bragg angle of 27.3°.
Patent Literature 4: JP-A-2001-296676 discloses a positively charged-type electrophotographic photoreceptor, which comprises a photoreceptor layer comprising a conductive substrate and a crystalline titanylphthalocyanine on the conductive substrate which has a maximum diffraction peak at Bragg angle (2θ±0.2°) of 27.3°, in X-ray diffraction with CuK α-ray. The electrophotographic photoreceptor is subjected to an exposure with a monochromatic light having wavelength of 600 nm or less to form an image.